Enhancement Of Electron Injection To Improve Efficiency Of Organic Light-Emitting Diodes

Organic light-emitting diodes?OLEDs?have been gained as the next generation of dream flat-panel solid state displays due to their several advantages such as a low driving voltage,low-power consumption,wide working temperature,the ultra-thin thickness,the possibility to produce flexible displays and so on.Since first reported by C.W.Tang in 1987,OLEDs have made great progress in materials and the improvements of the device properties.But in terms of luminous efficiency,OLEDs had been still inferior to the inorganic light-emitting diodes.In this paper,a series of OLEDs based on bipolar fluorescent materials or modified by photocathode materials CdS have been fabricated to improve the injection of electrons and balance the carriers for the better efficiency of OLEDs.Firstly,the new blue fluorescent OLEDs were fabricated based on Diphenyl-[4'-?1-phenyl-1H-benzoimidazol-2-yl?biphenyl-4-yl]-amine?TPABBI?and Diphenyl-[4'-?1-phenyl-1H-benzoimidazol-2-yl?-biphenyl-3-yl]-amine?TPAMI?,which are the bipolar materials with triphenylanmine and benzoimidazole.The OLED with the structure of ITO/MoO₃?10 nm?/NPB?80 nm?/TCTA?5 nm?/TPABBI?20 nm?/TPBi?40 nm?/LiF?0.5 nm?/Al?100 nm?possess the good photoelectric properties with the emission peak at a location of 464 nm,the chromaticity coordinates?CIE?coordinates of?0.15,0.14?,the maximum brightness of18970 cd/m² and the maximum current efficiency of 5.48 cd/A at 16 V.When x=15 nm,the emission peak of the OLED with the structure of ITO/NPB?40 nm?/TCTA?5 nm?/TPAMI?x nm?/TPBi?20 nm?/LiF?0.5 nm?/Al?100 nm??x=10,15,20,25?located at444 nm with the CIE coordinates of?0.17,0.17?.The maximum brightness and the maximum current efficiency of the blue OLED were 3080 cd/m² and 1.31 cd/A,respectively.Moreover,the color coordinates is a slightly changes with increasing the working voltage.The two materials can emit deep blue emission,especially,owned not only the hole-transporting but also the electron-transporting ability,which attributed to the structures of molecular with the donor?D?and acceptor?A?.The D-A molecular make the carriers inject to the OLEDs easily,improving the injection of electrons and balance the carriers for the better efficiency.Furthermore,D-A molecule would have great significance to simplify preparation technology of the OLEDs and addvariety of blue electroluminescence materials.Secondly,the green OLEDs were fabricated modified by CdS.The thickness of CdS,the position of CdS and CdS doping structure in OLEDs have been studied.?1?The injection of electrons enhanced when the layer of CdS inserted electron-transporting and cathode LiF/Al.The OLEDs with the structure of ITO/NPBX?50 nm?/Alq₃?50 nm?/CdS?x nm?/LiF?0.5 nm?/Al?100 nm??x=0,0.3,0.5,0.7,0.9?were fabricated.The OLED for x=0.5 nm exhibits the best properties with the maximum brightness of 3439 cd/m² and the maximum current efficiency of 1.66 cd/A,which are increased 16.14%and 6.41%compared to the OLED without CdS,respectively.?2?The OLEDs with the structure of ITO/2T-NATA?20 nm?/NPBX?50 nm?/C-545?0.2 nm?/Alq₃?50 nm?/CdS?0.5 nm?/LiF?0.5 nm?/Al?100 nm?were fabricated by changing the position of the 0.5 nm-thick CdS.It is shows that the significantly enhancement of electron injection and luminous efficiency when the 0.5 nm-thick CdS is inserted into between Alq₃ and LiF/Al.The maximum brightness of 6765 cd/m² at 16 V and the maximum current efficiency of 2.85 cd/A at 11 V are achieved,respectively.?3?The OLED doped CdS with the structure of ITO/MoO₃?10 nm?/NPB?30 nm?/Alq₃?45 nm?/Alq₃:10%CdS?15 nm?/LiF?0.5 nm?/Al?100 nm?have the maximum luminance of 32340 cd/m² at 12 V and the maximum current efficiency of 5.92 cd/A at 11.5 V.It indicated that CdS as a photocathode material which can absorb the light then generated light electronics injecting to the OLED again.In fact,the electrons injection enhanced and the efficiency of the OLEDs increased.Thirdly,the influence of the photocathode material used in the bipolar blue OLEDs and phosphorescent blue OLEDs modified by CdS have been studied.The bipolar blue OLED with the structure of ITO/MoO₃?10 nm?/TPABBI?35 nm?/Bphen?40 nm?/CdS?0.3 nm?/LiF?0.5 nm?/Al?100 nm?have the maximum luminance of 13590 cd/m² and the maximum current efficiency of 3.42 cd/A at 9V,the reduced efficiency roll-off as well.In the phosphorescent blue OLEDs with the structure of ITO/MoO₃?3 nm?/NPB?40 nm?/MCP:8%Firpic?30 nm?/Bphen?40 nm?/CdS?x nm?/LiF?0.5 nm?/Al?100 nm?,when x=0.3 nm,the maximum luminance of 29980 cd/m² at 16.5 V and the maximum current efficiency of18.89 cd/A at 11.5V,in which the efficieny increasead 9.3%compared with the OLED without CdS?16.56 cd/A?at the brightness of 20000 cd/m².The OLED modified by CdS would be an important reference for the OLEDs fabricated by inorganic materials and organic materials,especially for improving the properties of blue OLEDs.Finally,the white OLEDs?WOLEDs?were fabricated incorporating different phosphorescent blue dopants such as Firpic or Fir6 doped in MCP or CBP,and changed the position of the red emitting layer and the blue emitting layer to study the influence for the WOLEDs.The WOLED with the structure of ITO/NPB?40 nm?/MCP:8%Firpic?25 nm?/CBP:1%Ir?piq?₂?acac??5 nm?/Bphen?40 nm?/CdS?0.3 nm?/LiF?0.5 nm?/Al?100 nm?owed the maximum current efficiency of 12.01 cd/A at 12 V,the maximum luminance of16850 cd/m² at 23 V,the CIE coordinates nearby?0.34,0.37?in white region.Based on this,the other two groups of OLEDs were fabricated in which the position of the red emitting layer and the blue emitting layer changed each other.It is indicates that the WOLEDs of the red emitting layer near to the CdS thin layer owed the maximum luminance of 19590 cd/m² at 19 V,the maximum current efficiency of 10.06 cd/A at 18 V,and almost there was a little change for the efficiency.The WOLEDs of the blue emitting layer near to the CdS thin layer owed the maximum luminance of 22630 cd/m² at 18.5 V,the maximum current efficiency of 23.3 cd/A at 7 V,and there was a very large change for the efficiency.By analyzed the electroluminescence?EL?intensity of the two groups of devices,it was found that the color could be adjusted for the WOLEDs by modified the CdS thin layer based on the different emission conditions from the red emittihg materials and the blue emitting materials.The devices in this paper,when the red emitting layer near to the CdS thin layer,the electrons injected to the red emitting layer enhanced,which would be benefit for the red emitting lays and balance the emission from the two emitting layers.Therefore,the way for OLEDs modified by CdS not only can enhance the electrons injection,balance carriers and improve the efficiency,but also can adjust the CIE coordinates of WOLEDs.In a word,design the new materials combines many functions,optimize the structures of OLEDs for the best properties and lower the price,all of these would be the key for the industrialization of OLEDs.In view of the material science and nano-electronic devices,the influences and its mechanism had been studied about the electrons injection enhanced.